Automatic transmissions include a hydraulic power transmission having, on a torque delivery path between a power source and a transmission, a fluid coupling or a torque converter which is configured to transmit a torque of the power source continuously from a stall state to a directly connected state of an output shaft of the power source and an input shaft of the transmission. A known torque converter includes a lock-up clutch which directly connects a pump impeller and a turbine runner to eliminate a rotational speed difference therebetween when a rotational speed difference between the pump impeller and the turbine runner is small in order to enhance fuel economy during an operation of a vehicle. The lock-up clutch is controlled to be engaged or disengaged by a hydraulic pressure control of the hydraulic pressure control apparatus.
Known hydraulic pressure control apparatuses disclose technologies which stabilizes a hydraulic pressure outputted from an oil pump in an idling state of a vehicle. For example, JPH-06-147311A (hereinafter referred to as Patent document 1) discloses a known hydraulic pressure control apparatus which includes an oil pump driven by an engine, a pressure regulator valve which regulates oil outputted from the oil pump to be a level of target line pressure in accordance with a vehicle state, a lock-up control valve which introduces the line pressure to a lock-up chamber of a torque converter with a lock-up mechanism to establish an engaged state of the lockup mechanism and releasing an excessive oil to a circulation system via an oil cooler, and a circulation system fluid amount restricting means which reduces a dimension of an oil path at a position downstream from the lock-up control valve and upstream from the oil cooler when an amount of the oil outputted from the oil pump is reduced and an actual line pressure is reduced to be lower than the target line pressure. JP2000-54877A (hereinafter referred to as Patent reference 2) discloses an apparatus for an automatic transmission which is configured to transmit a rotational torque from an engine by an engagement of a frictional element for vehicle start by means of a hydraulic pressure operation during a gear selecting operation from a non-traveling range to a traveling range, which is configured to increase engine rotation speed in an idling state when an engine rotation speed does not decline in response to the engagement of the frictional element for vehicle start in a predetermined time from the gear selecting operation.
According to the construction of the hydraulic pressure control apparatus disclosed in Patent reference 1, in order to reduce the oil to be outputted from the lock-up control valve by the excessive amount, the circulation system fluid amount restricting means is additionally required, which increases a manufacturing cost. Further, according to the apparatus disclosed in Patent reference 2, in order to compensate for a lack of outputted amount of the oil from the oil pump, a control for increasing an engine rotation speed is performed during the gear selecting operation. With this construction, because the lack of the outputted amount of the oil from the oil pump is not overcome only by the automatic transmission, not only a control for the automatic transmission but also a control for the engine is complicated and a manufacturing cost increases.
A need thus exists for a hydraulic pressure control apparatus for a torque converter which is not susceptible to the drawback mentioned above.